1. Field of the Invention
This invention relates to a high operating speed type bipolar transistor in which a base region of a second conductivity type is formed in a collector region of a first conductivity type and an emitter region of a first conductivity type is formed in the base region. More particularly, it relates to a bipolar transistor in which the base region is formed by a graft base region, a linking base region and an intrinsic base region.
2. Description of Prior Art
For producing a high speed high performance bipolar transistor, it is necessary to reduce the base width W.sub.B, while it is also important to reduce the base resistance R.sub.bb'.
As the production method for realization of this high performance, the present applicant has proposed a technique of forming a linking base region between an intrinsic base region as an active region and a graft base region which is produced by diffusing impurities from the base contact electrode. This technique is disclosed for example in the Japanese Patent Application Nos. 184898/1987, 188025/1987 and 188026/1987.
Meanwhile, in the technique of linking the intrinsic base region and the graft base region by the linking base region of the same conductivity type, there are caused damages when the linking base region is formed by ion implantation. These damages by ion implantation give rise to accelerated diffusion into the active region and to adverse effects due to so-called channeling tail. Among these adverse effects is the increase in the base junction depth that is caused in the course of the ensuring heat treatment. As described in the aforementioned Japanese Patent Application No. 184898/1987, there is also known a method comprising forming side walls on the lateral sides of the base contact electrode and utilizing these side walls to remove the substrate partially to form an emitter region and a base region of extreme small size and, above all, extremely shallow depth. However, the side injection effect of the emitter and the base presents itself to interfere with the high speed operation.
As a technique overcoming this inconvenience, the present applicant has already proposed in the Japanese Patent Application No. 33686/1988 a technique according to which a diffusion suppressing or restraining region having an opposite conductivity type to that of the base region is provided below the intrinsic base region.
However, for further elevating the performance of the bipolar transistor, it is necessary to prevent the width of, for example, the linking base region functioning as a parasitic base region from being increased, to suppress the side injection effect more completely.